1) Are blow down valves same as depressuring valves? They are provided as bypass to PSVs.

2) Under what scenarios, are blowdown./ depressuring valves ( if same) required on LPG spheres? In my current project, the spheres have all three - PSVs, PCVs and blowdown valves. However, in one of the previous projects, there were just PSVs and PCVs.

Following notes can help query to some extent.1. Posts (written or going to be) of the thread "Pcv on spheres", http://www.cheresou...cv-on-spheres/ ,are judged useful. Among them, post No 26 makes reference to http://www.cheresou...-valve-as-bdv/ .1α. Post No 24 contains a web reference reporting several cases of depressurization (para 4.1, depressurization). Reference to API 2510 A is also made.2. In Greek refineries, depressurization of LPG spheres has not been installed. Even mandatory by mentioned web reference, API 2510 A is skeptical on sphere depressurizing and does not dictate it.However it has been installed (2009) on deethanizer column and splitter column of LPG recovery unit, using depressurizing valves by METSO. Discharge from depressurizing valve is connected to the PSV discharge, looking as bypass (but it is not really).3. Depressurizing ball valves are known to have a restriction orifice downstream, which does not occur in METSO valves.Note: Mentioned depressurizing in Greek refinery covers fire case too. Deethanizer (dia=1.1 m, 38 trays)depressurizing system results in 14" discharge pipe to flare header. The valve opens more and more during fire to lower operating pressure from 22 to 6 barg in ~15 min.API 2510 A, 2.11.3: "..This may, in some cases, be total flame envelopment of the vessel. Such a situation can result in large depressurizing rates requiring a large-size depressurizing system". Such a system on an LPG sphere would be of bigger size than that of deethanizer, so in a way it is not feasible. But this would help avoiding BLEVE of the sphere during fire, even if the sphere itself is destroyed.

Thanks. That was very informative. Also, the lastest API 2510, 8th Edition, May 2001 does not mention the following earlier statement in API publication 2510 A (Dec 1996), para 2.11 and 6.6.7.3. The new API 2540 simply directs to API 521 on depressurization subject.

"Generally, vapor depressurizing systems appear to have limited application in LPG storage". This may be due to "requiring a large-size depressurizing system", on the other hand this can avoid BLEVE since "after approximately 10-30 minutes of direct flame exposure, the vessel usually ruptures violently" (conditions per para 6.6.3.1.1).

@EveryonePlease can anyone suggest a possible reason, for my Query 2 in the original post. I was thinking may be BDV was not shown as the project was in FEED stage and left it to the EPC contractor. But neither do I see any NOTE on the P&ID which confirms my thought. So not completely sure.

1. Thanks for the information contained in API 2510 (8th ed, 2001) and API 2540 .2. I do not understand the question fully, a try to cover it is as follows.α. Scenarios of depressurizing are reported in http://www.scandpowe...m191-203082.pdf '> http://www.scandpowe...m191-203082.pdf , chapter 4.1 - Depressurization, first three paragraphs.β. Blowdown valves connected to PSV discharge are used for depressurizing, I think. Most probably they cover the fire case. Unusually big size of line downstream the blowdown valve (as well as orifice) indicates this. Valves are assumed remote operated.γ. Probably previous project did not consider depressurization, not mandatory for spheres at least by previous API 2510 A (1996). It is considered in the new project, since safety requirements get stricter and stricter.

I found a flow chart in chapter 12 of "Handbook of Fire and Explosion Protection Engineering Principles" which reflects requirement of depressurization system on fire duration and rupture time as per API 521, Fig. 2. The examples in the book show that if rupture time is in several hours, then depressurization is not required.

Probably, thats why it was not in previous project and the current client is more safety consicious and thats why it has been added.

1) Are blow down valves same as depressuring valves? They are provided as bypass to PSVs.

Yes you can use the valve of blowdown for depressurization too. Usually the blowdown is not active at the same time of the PSV´s because the philosophy are different.

2) Under what scenarios, are blowdown./ depressuring valves ( if same) required on LPG spheres? In my current project, the spheres have all three - PSVs, PCVs and blowdown valves. However, in one of the previous projects, there were just PSVs and PCVs.

The blowdown is used in case of external fire and is activated manually from control room or local button. The depressuring is usually used during the maintenance.
In the natural gas fires are not equivalent to liquid hydrocarbon pool fires. Natural gas fire protection and mitigation requires different protection methods than for those used for fighting liquid hydrocarbon pool fires, which can be extinguished using a fire water system or a foam system. . It is standard natural gas industry practice to isolate the hydrocarbon gas sources to the facility and evacuate all personnel from the facility. Once the source of the gas is isolated, the feed to the fire is terminated and the fire is quickly extinguished from lack of fuel.

In addition, consideration for required depressurization time, resulting Flare Header temperatures, and depressurization control schemes must be given close attention. These systems can be highly complex due to the transient nature of the process and require careful design procedures to ensure a safe Depressurization System.

From my point of view the depressurization is more controlled in addition to the increase in costs in the flare system that has the use blowdown.
Finally, my recommendation is better analyze the use of blowdown if you have a PCV valve.